1. Field of the Invention
The present invention relates to a color ink-jet printer capable of ejecting ink droplets of different colors.
2. Discussion of Related Art
U.S. Pat. No. 6,416,149 B2 (in particular, FIGS. 4-6, and col. 1, lines 46-53 and col. 5, lines 43-50) corresponding to JP-2001-301206A discloses an ink-jet printer operable such that at least one droplet of an ink each having a predetermined volume is ejected from each nozzle, so as to form a dot of ink on a recording medium in the form of a paper sheet, such that where a plurality of ink droplets are ejected from the nozzle, the ink droplets overlap each other so as to form one ink dot. Thus, a desired gray-scale value can be established at each picture element of an image corresponding to each ink dot to be formed on the paper sheet according to image data (printing data), by suitably selecting one of three different total volume values (large, medium and small values) of the above-indicated at least one ink droplet, for each of the ink dots, so that each ink dot has the corresponding one of three different sizes or diameters which are determined by the respective three different total volume values.
If the technique disclosed in the above-identified U.S. Patent is applied to a color ink-jet printer having a plurality of rows of nozzles that are arranged to eject droplets of inks of respective different colors (e.g., yellow [Y], magenta [M], cyan [C] and black [B]), the same number of the ink droplets corresponding to one ink dot are ejected for each of the different colors, that is, the total volume of the ink droplets corresponding to one ink dot is the same for all of the different colors, when the gray-scale values at picture elements corresponding to the four colors are equal to each other. However, the inks of different colors have different compositions including different coloring agents and having different drying speeds, and the viscosity of the ink having a relatively high drying speed may be excessively increased at the meniscus surface of the ink remaining in a given nozzle, due to evaporation of an aqueous component of the ink at the meniscus surface, which takes place if the ejection of the ink droplets from that nozzle is absent for a relatively long time. In this case, the nozzle may suffer from so-called “plugging” due to increased viscosity of the ink at the meniscus surface, particularly when the ink droplets ejected last from that nozzle to form the last ink dot have a relatively small volume. This plugging may lead to a failure to subsequently eject at least the first one of the droplets to be ejected from the plugged nozzle to form the next ink dot. In this instance, the ink dot is not formed at a predetermined point on the paper sheet, resulting deterioration of quality of an image printed on the paper sheet.
The aspect indicated above will be described in detail by reference to FIG. 8, which shows an example of an arrangement of ink dots of four colors (Y, M, C and K) to be formed on a paper sheet according to print data, where the yellow ink has the highest drying speed. In this example, all picture elements corresponding to the respective ink dots have the same gray-scale value according to the print data, which value corresponds to only one ink droplet of a relatively small volume to be ejected from the nozzle. Further, the operations to eject the ink dots at the respective picture elements are commanded a short time after the moment of initiation of a printing operation after a relatively long non-ink-ejection period. Solid-line circles in FIG. 8 indicate the magenta, cyan and black ink dots which have been actually formed on the paper sheet, while broken-line circles in FIG. 8 indicate the yellow ink dots which have not been actually formed on the paper sheet, due to a failure of ejection of the yellow ink droplets from the respective nozzles of the ink-jet head corresponding to the yellow ink. Namely, these nozzles corresponding to the yellow ink were plugged with the dried yellow ink during the relatively long non-ink-ejection period, so that the yellow ink droplets of the relative small volume can not be subsequently ejected from the plugged nozzles. In this case, a local area corresponding to the picture elements for the yellow ink dots remains blank on the paper sheet, resulting in deterioration of quality of the image printed on the paper sheet.